This invention relates generally to the welding of thin metal parts and particularly to a welding apparatus for welding a washer to the support spring of a shadow mask assembly for color kinescopes.
A color television tube contains an apertured color selection electrode, commonly called a shadow mask, which causes the three electron beams to strike the phosphors which emit the proper colors of light. As shown in FIGS. 1-3, a shadow mask 11 is permanently attached to a frame 12. The frame 12 provides rigidity for the shadow mask 11 and serves as a mechanism for supporting the shadow mask in a faceplate panel 13. A phosphor screen 14 is provided on the inside surface of the panel 13 so that the panel serves as the viewing screen of the tube. The shadow mask 11 is supported a specified distance, commonly called the q spacing, from the screen 14. Metallic conically shaped studs 16, 17, 18 and 19 are embedded in the four sidewalls 15 of the panel 13 (FIG. 1). Resilient leaf springs 21, 22, 23 and 24 are welded to the shadow mask frame 12. The springs 21-24 each contain an aperture 26 (FIG. 3) which engages the studs 16-19 to retain the shadow mask 11 in the panel 13 at the required q spacing.
The proper engagement of the studs and spring apertures necessitates the accurate location of the studs 16-19 in the sidewalls of the panel and the accurate location of the springs on the frame 12. It is difficult to accurately locate all four of the studs in the panel 13 because the glass is molten when the studs are inserted and changes in the glass during cooling are inconsistent and unpredictable. Accordingly, it has become standard practice to provide one of the leaf springs with an oversized aperture so that the aperture is larger than the stud and does not engage the stud. The other three springs include apertures which are dimensioned to firmly engage the studs. A washer 27 (FIG. 1) is placed on the stud 18 to rest against the spring 23 which has the oversized aperture. The aperture in the washer 27 is dimensioned to snugly engage the stud 18 and the washer diameter is sufficiently large to completely cover the oversized aperture. The frame 12 is placed in the panel 13 with the other three springs snugly engaging studs irrespective of whether or not the stud 18 is off-center from the oversized aperture in the spring 23. The washer is then welded to the spring to accurately locate the fourth spring aperture with the fourth stud. Such a method is disclosed in U.S. Pat. No. 3,334,259.
Difficulty has arisen in this technique because the limited space between the frame 12 and the sidewall 15 of the panel 13 makes it very difficult to weld the washer 27 onto the spring 23. Accordingly, typically, the washer 27 is lightly welded to the spring 23. The shadow mask is removed from the panel 13 and the washer is permanently welded to the spring. Problems arise because the initial light weld frequently breaks and the washer 27 separates from the spring 23. The shadow mask must be put back into the panel and initial welding repeated. There, therefore, is a need for a welding apparatus which permits the permanent and strong welding of the washer 27 to the spring 23 while the frame 12 is positioned in the faceplate 13. The instant invention fulfills this need.